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According to RMT, Night Tube operators "have been prevented from moving into vacant full time positions for at least 18 months", despite other tube station staff being able to apply.

The issue of overtime has also been raised, as operators do not get overtime pay until they go over 35 hours of work a week, despite their contract saying they work 16 hours.

The first night tube was launched in August 2016

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Both these issues have been considered by RMT’s National Executive Committee, which has taken the decision to ballot all RMT Night Tube train operator members for strike action and action short of a strike.

RMT general secretary, Mick Cash, said: "It is outrageous that London Underground have decided to discriminate against their Night Tube drivers.

"This is a senseless and damaging policy that picks out one group of staff for negative treatment and of course the drivers are angry and that is why we are balloting for action.

Passengers using the night tube will be affected if the March 14 ballot closes and staff have voted for strike action (Photo by Jack Taylor/Getty Images) (Image: 2016 Getty Images)

"RMT would call on LU to see sense, stop these stupid attacks on career progression and fair reward for overtime working and confirm that this discrimination against the Night Tube drivers has been lifted.”

Peter McNaught, London Underground operations director said in response: "Night Tube services have been running since August last year in line with agreements reached with the unions.

"We invite the RMT to meet with us to discuss any issues that need to be resolved and there is no need to threaten industrial action."